Generally, a semiconductor element and an electrode are held by pressure through a pan spring in a pressure-contact type semiconductor device so as to ease the conductivity of electricity and heat. Prepared pressure holding mechanisms include a screw type, a snap ring type, and a mechanism disclosed in Japanese Published Patent specification Sho. No. 46-35213 in which several points of a case are broken to produce projections which push down the pan spring to maintain the pressure.
A diode as such a pressure-contact type prior art semiconductor device will be described in the following with reference to FIG. 1:
The reference numeral 1 designates a semiconductor element. The numeral 2 designates a case comprising a support portion 2a, a cylindrical body portion 2b produced integrally therewith, and a screw portion 2c produced below the support portion 2a. This case 2 constitutes an anode. The numeral 3 designates an electrode rod which constitutes a cathode. The numeral 6 designates a pan spring. The numeral 4 designates an insulating washer. The semiconductor element 1 is mounted on the support portion 2a of the case 2, and the electrode rod 3 is mounted on the semiconductor element 1. The insulating washer 4, the washer 5, and the pan springs 6 are provided through the electrode rod 3.
The production method of this device is described with reference to FIG. 2:
A predetermined load is applied to the pan spring 6 by a pressure applying apparatus 10 in the direction of arrow C, and three bites 11 (two are shown in FIG. 2) arranged outside the case 2 are operated as shown in dotted lines in the directions of arrows D and E, respectively, thereby breaking the outside wall of the case 2 to produce projections 12 inside thereof. Then, it is necessary to provide a small gap (h) between the upper surface of the pan spring 6 and the lower surface of the bite 11 because it is difficult to produce projections 12 so as to be in closely contact with the upper surface of the pan spring 6. However, if this gap (h) becomes close to 0, the pan springs 6 and/or the bite 11 are broken. On the other hand, if the gap (h) has a some value, the bending dimension of the pan spring 6 varies and the spring load is reduced to a great extent.
When the pressure applying apparatus 10 is removed from the upper surface of the pan spring 6 after the projections 12 are produced, the upper portions of the pan springs 6 come into contact with the lower ends of the projections 12. Then, the projections 12 may deform in the upward direction by the reactive spring force of the pan springs 6, and the bending dimension of the pan-spring 6 varies similarly as described above, thereby reducing the spring load to a greater extent. Hereupon, the pressure applied to the semiconductor elment 1 is designed at about 100 kg/cm.sup.2, taking into consideration the electrical and thermal resistances.
Furthermore, when the case 2 is broken by the bites 11, broken pieces 13 are generated from the case 2. These broken pieces 13 fall down on the insulating washer 4, and can short-circuit the washer 4 and the electrode rod 3 which are insulated with the case 2, thereby lowering the quality of the electrical characteristics.
In this type of mechanism for holding the semiconductor element, it is impossible to keep the load of the pan spring 6 at a constant level, and the broken pieces 13 fall down separated from the case 2, thereby lowering the electrical characteristics.